1. Field of the Invention
This invention relates to an ink jet printer. More particularly, this invention relates, in an ink jet printer, to a circuit for selectivery ink droplets based on image data.
2. Description of the Prior Art
U.S. Pat. No. 4,673,951 discloses a Hertz-type ink jet printer which is provided with an ink discharge nozzle with a piezoelectric oscillator. The number of oscillations imparted by the oscillator to the nozzle is fixed at a prescribed magnitude by the characteristics of ink, the inner diameter of nozzle orifice, and the flow rate of ink. By the oscillations thus imparted to the nozzle, the ink spouted from the nozzle is caused to fly in the form of minute droplets of a fixed diameter spaced at fixed intervals toward a recording paper. Inside this nozzle is disposed a charging electrode to which charging signals based on image data are fed.
A control electrode is interposed between a rotary drum for holding the recording paper and the nozzle. This control electrode is provided with a deflection electrode and a grounding electrode. Between the deflection electrode and the grounding electrode, an electric field for deflection is formed in the direction perpendicular to the axis of the nozzle. The row of ink droplets which has been spouted from the nozzle is propelled toward the record paper when it has not be charged by the charging electrode within the nozzle or is deflected by the electric field, trapped by the control electrode, and wasted when it has been charged.
In the ink jet printer described above, when the ink droplets spouted from the nozzle are propelled as arrayed linearly, they ought to produce an image of desired quality. Actually, however, the image formed on the record paper is observed to be blurred with minute ink dots other than desired ink dots. The occurrence of such very minute ink dots prevents production of an image of high quality. The inventors have conducted various experiments and researches with a view to improving the quality of an image to be formed on the recording paper, to find that the following cause can be adduced for the occurrence of these minute ink dots.
The leading end of the row of uncharged ink droplets destined to form a recorded image is caused under the electrical influence of the preceding row of charged ink droplets to be dislocated in the direction of the deflection electrode and, during the course of this dislocation, very minute ink dots or secondary dots inducive of impairment of image quality occur near the recording dots inherently used for recording.
After further experiments and researches, they have found that when the propelling speed of ink is heightened, the ink droplets in the trailing end part of the row of uncharged ink droplets to be used for recording deviate toward the grounding electrode side and the ink droplets in the trailing and part of the row of charged ink droplets deviate toward the deflection electrode. This behavior of such ink droplets may be logically explained by a postulate that a time lag occurs between the time the output voltage for charging is fed to the charging electrode within the nozzle and the time the ink is charged to the prescribed maximum level and, at the same time, a time lag also intervenes between the time the supply of the voltage to the charging electrode is stopped and the time the charge of the ink falls to the minimum level. These time lags give rise to ink droplets charged to an intermediate level in addition to the uncharged ink droplets and the ink droplets charged to the maximum level. These additionally formed ink droplets are deviated in a direction perpendicular to the axial direction of the nozzle under the influence of the electric field and the ink droplets thus deviated, during the course of the travel of deviation, give rise to secondary dots. When these secondary dots land on the recording paper, they induce impairment of an image or lines.